Heat shrinkable polypropylene laminate film

ABSTRACT

The heat shrinkable polypropylene film having an isotactic polypropylene layer as surface layers and a layer mainly composed of a syndiotactic polypropylene as an intermediate layer, having a shrinkage of not less than 25% at 100° C. in the machine and transverse directions and being stretched at least two times in the machine and transverse directions, respectively. The film has a good packaging machine applicability and excellent propagation tear resistance, low temperature shrinkability and heat resistance, and is suitably used as a shrink packaging material.

TECHNICAL FIELD

The present invention relates to a shrink packaging material, and moreparticularly to a heat shrinkable polypropylene laminate film having anintermediate layer mainly comprising a syndiotactic polypropylene andhaving a good packaging machine applicability and excellent propagationtear resistance, low temperature shrinkability and heat resistance.

BACKGROUND ART

Stretched films such as polyvinyl chloride films, polypropylene filmsand polyethylene films are hitherto known as heat shrinkable films.

Among them, heat shrinkable polypropylene films have been generally usedin high speed automatic packaging, since they have a good film firmessand are excellent in packaging machine applicability. However, ascompared with heat shrinkable polyvinyl chloride films, the heatshrinkable polypropylene films exhibit heat sealability and heatshrinkability only at fairly high temperatures. Moreover, thepropagation tear resistance and impact strength of the film are low, soit cannot necessarily be said that the heat shrinkable polypropylenefilms have a good applicability to packaging of heavy weight articlesand the like.

The above-mentioned heat shrinkable polypropylene films are the heatshrinkable films made of crystalline isotactic polypropylene which is analready known material, copolymers composed mainly of polypropylene suchas propylene-ethylene copolymer and propylene-ethylene-buteneterpolymer, and mixtures of these polymers with thermoplastic polymerscapable of forming a film when mixed with these polymers.

In order to eliminate the above-mentioned defects, particularly toimprove the heat sealability and the propagation tear resistance,packaging films using a syndiotactic polypropylene in the surface layersare reported in Japanese Patent Publication Kokai Nos. 5-77309, 5-200957and 5-245992.

However, though these proposals improve the heat sealability and thepropagation tear resistance, it cannot be said that the low temperatureshrinkability is satisfactory. Also, since the ratio of the syndiotacticpolypropylene layers to the whole layers is small, the effect on thepropagation tear resistance is small. Further, because the syndiotacticpolypropylene having a low melting point is used in the surface layers,the films have a defect of whitening if a package is shrinked at hightemperatures.

The present invention is intended to provide a film constitution whichimproves the low temperature heat shrinkability and the propagation tearresistance without deteriorating the packaging machine applicability andoptical property that conventional heat shrinkable films using apolypropylene resin possess, and moreover which has a wide range of heatshrinking temperature.

DISCLOSURE OF THE INVENTION

The present inventors have made intensive studies to solve theabove-mentioned problems, and as a result, they have found that a heatshrinkable polypropylene film having excellent packaging machineapplicability and low temperature heat shrinkability and an improvedpropagation tear resistance is obtained by using a specificpolypropylene resin and conducting coextrusion with setting a specificlayer thickness ratio. Thus they have arrived at the present invention.

That is to say, the heat shrinkable film of the present invention is apolypropylene laminate film stretched at least two times in both themachine and transverse directions, characterized in that the surfacelayers comprise an isotactic polypropylene and the intermediate layerconsists essentially of 50 to 100% by weight of a syndiotacticpolypropylene and 0 to 50% by weight of an isotactic polypropylene, andthat the thickness of the intermediate layer is not less than 30% of thetotal thickness of all layers, the thickness of each of the surfacelayers is at least 1 μm, and the shrinkage of the film at 100° C. is notless than 25% in both the machine and transverse directions.

The isotactic polypropylene used in the surface layers is known as acrystalline polypropylene, and includes a propylene-ethylene copolymerand a propylene-ethylene-butene terpolymer which are composed mainly ofan isotactic polypropylene and have not more than 15% by weight ofn-heptane-soluble fraction, and mixtures of these polymers with athermoplastic polymer capable of forming a film when mixed with thesepolymers. The propylene-ethylene copolymer and thepropylene-ethylene-butene terpolymer are those containing 2 to 10% byweight of units of ethylene or units of ethylene and butene. As thethermoplastic polymer, there can be used an ethylene-vinyl acetatecopolymer, an ethylene-α-olefin copolymer, a propylene-butene copolymer,an ionomer, a polybutene, a petroleum resin and the like within therange not hindering objects of the present invention.

The intermediate layer must be a layer essentially containing asyndiotactic polypropylene as a main component, namely must have a resincomposition of 50 to 100% by weight of the syndiotactic polypropyleneand 0 to 50% by weight of the isotactic polypropylene. If thesyndiotactic polypropylene is not more than 50% by weight and theisotactic polypropylene is not less than 50% by weight, a proper levelof the heat shrinkage is not exhibited at low temperatures and further,the effect of improving the propagation tear resistance becomes small.

The isotactic polypropylene used as one of the components of theintermediate layer substantially corresponds to the above-mentionedisotactic polypropylene in the surface layers, but it is not alwaysnecessary that the isotactic polypropylene mixed in the intermediatelayer is the same as the isotactic polypropylene resin used in thesurface layers. On the other hand, the syndiotactic polypropylene is apolypropylene having a high syndiotacticity such that the syndiotacticpentad fraction measured by ¹³ C-NMR is not less than 0.7, unlike aconventional syndiotactic polypropylene having a low syndiotacticityobtained by using a vanadium catalyst. Such a syndiotactic polypropylenecan be obtained by using a catalyst composed of a co-catalyst and acrosslinking type transition metal compound having ligandsnon-symmetrical to each other as described in, for example, JapanesePatent Publication Kokai Nos. 2-41303, 2-41305, 2-274703, 2-274704,3-179005 and 3-179006. Also, the measurement of syndiotactic pentadfraction by ¹³ C-NMR can be made by a known method as described, forexample, in Japanese Patent Publication Kokai No. 2-41303.

When recovered films are recycled in the intermediate layer in additionto the above resin composition, it is a matter of course that theisotactic polypropylene used in the surface layers is incorporated intothe intermediate layer, and the intermediate layer may contain, inaddition to the above-mentioned resin composition, other resins, e.g.,an ethylene-vinyl acetate copolymer, an ethylene-α-oiefin copolymer, apropylene-butene copolymer, an ionomer, a polybutene, a petroleum resinand the like so long as the objects of the present invention are nothindered.

The thickness of the intermediate layer of the heat shrinkable laminatefilm of the present invention must be not less than 30%, preferably notless than 50%, of the total thickness. If the thickness ratio of theintermediate layer is less than 30%, the effect of improving thepropagation tear resistance is lowered remarkably and the effect on thelow temperature shrinkability is also lowered.

Further, the intermediate layer in the present invention is notnecessarily composed of one layer, and can be composed of two or morelayers as occasion demands. Also the thickness of the surface layerafter the stretching is required to be selected so as to be at least 1μm, preferably at least 2 μm. If the thickness of each surface layerafter the stretching is less than 1 μm, the film lacks firmness and theeffect of improving the packaging machine applicability in a high speedrange becomes small, and in addition, since the heat resistingtemperature is lowered, whitening of the film occurs, if shrinked athigh temperature, at the time of shrinking after packaging objects to bepackaged.

Also it is a matter of course that additives such as a lubricant, ananti-blocking agent, an antistatic agent and an anti-fogging agent maybe suitably used for the purpose of providing the respective usefulfunctions so long as the objects of the present invention are nothindered.

The heat shrinkable laminate films of the present invention are preparedby known stretching methods. As one example thereof, a detailedexplanation is given below taking the case of a tubular preparationmethod.

Firstly, a tubular non-stretched film prepared by melt-kneading in threeextruders and coextruding through a three-layered circular die so thatthe intermediate layer is made of a polypropylene resin comprising theabove-mentioned syndiotactic polypropylene resin as a main component andthe surface layers are made of an isotactic polypropylene resin, andonce solidifying by rapid cooling without stretching. The obtainedtubular non-stretched film is then fed to a tubular stretchingapparatus, for instance, as shown in FIG. 1, and is subjected to asimultaneous biaxial orientation by inflation stretching at least 2times, preferably at least 2.5 times, in both the machine and transversedirections within an orientable temperature range with application of agas pressure to the inside of the tube. The film taken out of thestretching apparatus can be annealed if necessary, and spontaneousshrinkage during the storage can be inhibited by this annealing.

The feature of the present invention resides in using the syndiotacticpolypropylene in the intermediate layer, thereby providing a film havinggood low temperature shrinkability and propagation tear resistance. Alsothe packaging machine applicability, optical property and heatresistance, which are advantages of the existing polypropylene films,are not impaired because the isotactic polypropylene used in the surfacelayers. Further, by laminating these layers, the temperature rangecapable of shrinking is widened to such an extent that the externalappearance of the film is not impaired, so the films easy to handle canbe provided.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic section view illustrating a tubular biaxialstretching apparatus used in Examples and Comparative Examples, wherein1 is a non-stretched film, 2 is low-speed nip rolls, 3 is high-speed niprolls, 4 is a preheater, 5 is a main heater, 6 is a cooling air ring and7 is collapser rolls.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is concretely explained by means of the followingExamples and Comparative Examples, but is not limited to these Examples.

The measurements shown in the Examples were made by the followingmethods.

1) Heat shrinkability

Measured according to JIS Z 1709

2) Propagation tear resistance

Measured according to JIS Z 1707

3) Packaging machine applicability

A stretched film was applied to a commercial high speed pillow typepackaging machine under the conditions that the bead sealing temperaturewas 220° C., the tunnel temperature was from 130° to 175° C., therunning speed of the objects to be packaged was 100 pieces/minute andthe shrinking tunnel passing time was 4 seconds. The evaluation was madeby a yield of good finished packages.

4) Temperature range for heat shrinking

A heat shrinking temperature range where a good packaging finish wasobtained, and a temperature range where whitening and melting of thefilm did not occur, were measured simultaneously with the packagingmachine applicability.

EXAMPLE 1

A syndiotactic polypropylene (product of Mitsui Toatsu Kagaku KabushikiKaisha having a syndiotacticity of 0.79 and an MI of 4.9 g/10 minutes)and an isotactic polypropylene were melt-kneaded separately in threeextruders at a temperature of 200° to 250° C. and co-extruded through athree-layer circular die kept at 220° C. to form a film consisting of anintermediate layer of 100% by weight of the syndiotactic polypropyleneand surface layers of 100% by weight of the isotactic polypropylene. Atthat time, the thickness proportion of the intermediate layer was set at75% of the total thickness of all layers. The co-extruded tubular moltenfilm was cooled to room temperature by sliding its inner side on theouter surface of a cylindrical cooling mandrel in which a 20° C. coolingwater was circulated, and by passing the outer side through a water bathto water-cool the film, and was taken out to give a tubularnon-stretched film having a thickness of 270 μm.

This tubular non-stretched film was used as a starting film and led to atubular biaxial stretching apparatus shown in FIG. 1 and stretched 5times in the machine direction and 4 times in the transverse directionat a temperature of 95° to 105° C. The stretched film taken out of thestretching apparatus was cooled to room temperature by a cooling airring, collapsed and wound up. The stability of the stretching tube wasgood, and also non-uniform stretching state was not observed.

The obtained stretched film had an average thickness of 15.0 μm, and thethickness of each of the surface layers was 1.9 μm. The heat shrinkageat 100° C. was 30% in both the machine and transverse directions, andthe propagation tear resistance was 10.3 g in the machine direction and11.1 g in the transverse direction. As a result of evaluation of thepackaging machine applicability, there were obtained heat-shrinkpackages having a tight feeling and soft corners within a heat shrinkingtemperature range where the shrinking tunnel temperature was set at atemperature of 135° to 170° C. Also there was no pin holes in the sealedportion of the obtained packages, and no breaking from an air releasehole occurred.

EXAMPLE 2

Stretching of a film was carried out under the same conditions as inExample 1 with the thickness proportion of the intermediate layer beingset at 75% of the total thickness of all layers except that the film wascomposed of an intermediate layer of 55% by weight of the syndiotacticpolypropylene used in Example 1 and 45% by weight of an isotacticpolypropylene and surface layers of 100% by weight of the isotacticpolypropylene. The stability of the stretching tube was good, andnon-uniform stretching state was not observed.

The obtained stretched film had an average thickness of 15.0 μm, and thethickness of each of the surface layers was 1.9 μm. The heat shrinkageat 100° C. was 29% in both the machine and transverse directions, andthe propagation tear resistance was 9.1 g in the machine direction and9.7 g in the transverse direction. As a result of evaluation of thepackaging machine applicability, there were obtained heat-shrinkpackages having a tight feeling and soft corners within a heat shrinkingtemperature range where the shrinking tunnel temperature was set at atemperature of 135° to 170° C. Also there was no pin holes in the sealedportion of the obtained packages, and no breaking from an air releasehole occurred.

EXAMPLE 3

Stretching of a film was carried out under the same conditions as inExample 1 except that the film was composed of an intermediate layer of100% by weight of the syndiotactic polypropylene used in Example 1 andsurface layers of 100% by weight of an isotactic polypropylene, and thethickness proportion of the intermediate layer was set at 50% of thetotal thickness of all layers. The stability of the stretching tube wasgood, and non-uniform stretching state was not observed.

The obtained stretched film had an average thickness of 15.0 μm, and thethickness of each of the surface layers was 3.8 μm. The heat shrinkageat 100° C. was 30% in both the machine and transverse directions, andthe propagation tear resistance was 9.6 g in the machine direction and10.3 g in the transverse direction. As a result of evaluation of thepackaging machine applicability, there were obtained heat-shrinkpackages having a tight feeling and soft corners within a heat shrinkingtemperature range where the shrinking tunnel temperature was set at atemperature of 140° to 170° C. Also there was no pin holes in the sealedportion of the obtained packages, and no breaking from an air releasehole occurred.

COMPARATIVE EXAMPLE 1

Stretching of a film was carried out under the same conditions. as inExample 1 with the thickness proportion of the intermediate layer beingset at 75% of the total thickness of all layers except that the film wascomposed of an intermediate layer of 45% by weight of the syndiotacticpolypropylene used in Example 1 and 55% by weight of an isotacticpolypropylene and surface layers of 100% by weight of the isotacticpolypropylene. The stability of the stretching tube was good, andnon-uniform stretching state was not observed.

The obtained stretched film had an average thickness of 15.0 μm, and thethickness of each of the surface layers was 1.9 μm. The heat shrinkageat 100° C. was 20% in both the machine and transverse directions, andthe propagation tear resistance was 6.6 g in the machine direction and5.3 g in the transverse direction. As a result of evaluation of thepackaging machine applicability, shrinking did not sufficiently occur ata shrinking tunnel temperature of 150° C., thus giving packages having apoor appearance and stiff corners. Further, there occurred breaking froman air release hole due to rubbing of the packages with each other. Alsothe breaking occurred at a sealed part at a shrinking tunnel temperatureof 170° C.

COMPARATIVE EXAMPLE 2

Stretching of a film was carried out under the same conditions as inExample 1 except that the film was composed of an intermediate layer of100% by weight of the syndiotactic polypropylene used in Example 1 andsurface layers of 100% by weight of an isotactic polypropylene and thethickness proportion of the intermediate layer was set at 25% of thetotal thickness of all layers. The stability of the stretching tube wasgood, and non-uniform stretching state was not observed.

The obtained stretched film had an average thickness of 15.0 μm, and thethickness of each of the surface layers was 5.6 μm. The heat shrinkageat 100° C. was 18% in the machine direction and 20% in the transversedirection, and the propagation tear resistance was 3.3 g in the machinedirection and 2.3 g in the transverse direction. As a result ofevaluation of the packaging machine applicability, shrinking did notsufficiently occur at a shrinking tunnel temperature of 150° C., thusgiving packages having a poor appearance and stiff corners. Further,there occurred breaking from an air release hole due to rubbing of thepackages with each other. Also the breaking occurred at a sealed part ata shrinking tunnel temperature of 170° C.

COMPARATIVE EXAMPLE 3

Stretching of a film was carried out under the same conditions as inExample 1 except that the film was composed of an intermediate layer of100% by weight of the syndiotactic polypropylene used in Example 1 andsurface layers of 100% by weight of an isotactic polypropylene and thethickness proportion of the intermediate layer was set at 90% of thetotal thickness of all layers. The stability of the stretching tube wasgood, and non-uniform stretching state was not observed.

The obtained stretched film had an average thickness of 15.0 μm, and thethickness of each of the surface layers was 0.75 μm. The heat shrinkageat 100° C. was 33% in both the machine and transverse directions, andthe propagation tear resistance was 9.0 g in the machine direction and9.3 g in the transverse direction. As a result of evaluation of thepackaging machine applicability, the film lacked firmness and therunning of the film at the packaging machine became bad, so continuoushigh speed packaging was impossible. Though heat shrink packages havinga tight feeling can be obtained at a shrinking tunnel temperature of 135° C., whitening of the film occurred at a temperature of not less than150° C., thus giving packages having a poor appearance.

INDUSTRIAL APPLICABILITY

As mentioned above, the heat shrinkable polypropylene laminate film ofthe present invention has layers made of raw materials satisfying therespective specific conditions, so it has a good low temperatureshrinkability and a good propagation tear resistance and in addition, isexcellent in applicability to high speed packaging machines and heatresistance.

We claim:
 1. A heat shrinkable polypropylene laminate film comprisingsurface layers and an intermediate layer and stretched at least twotimes in both the machine and transverse directions, wherein saidsurface layers comprise an isotactic polypropylene and said intermediatelayer consists essentially of 50 to 100% by weight of a syndiotacticpolypropylene and 0 to 50% by weight of an isotactic polypropylene, andwherein the thickness of said intermediate layer is at least 30% of thetotal thickness, the thickness of each of said surface layers is atleast 1 μm, and the shrinkage at 100° C. is at least 25% in the machineand transverse directions.
 2. The laminate film of claim 1, wherein thesyndiotactic pentad fraction measured by ¹³ C-NMR of said syndiotacticpolypropylene in said intermediate layer is not less than 0.7.